I can say one thing for sure that airtel data card sucks big time especially in those areas where edge is not supported. You would have a hard time opening even google home page leave alone larger pages.
I have been using the reliance data card for more then two weeks now and I am very happy with its performance both in and out of new delhi. I have even been to some very remote areas and to my surprise it still works very well even where airtel doesnt have its network signal atall..